This invention relates to plaster molding compositions and, more particularly, to a suitable molding composition to replace the gypsum-fibrous talc-sand mixtures commonly used in preparing metal castings molds.
Low temperature fusion metals, such as aluminum, have for generations been cast in disposable plaster molds made primarily from calcined gypsum plaster (i.e., plaster of paris) and a fibrous talc (an asbestos like material). While even as early as around 1900, there were suggested improvements (see for example, U.S. Pat. Nos. 816,833 and 821,718), the commercial mix has remained essentially unchanged to the present day.
Typically, these fine castings molding compositions comprise around 50% gypsum plaster, 30% fibrous talc, 19% sand, 1% Portland cement and a small amount of an accelerator such as Terra Alba. Such a plaster mold composition is ideally suited to an end use of this particular type.
That end use involves (1) dry blending of the components into a molding composition, (2) slurrying the dry blend with water, (3) pouring the slurry around a master pattern, (4) stripping the hardened wet mold from the master, (5) trimming and patching as necessary, (6) drying in an oven, (7) gating and inserting chills (8) pouring the molten metal such as aluminum in the mold, (9) cooling, (10) separating the cast metal shape from the mold by using a high-pressure water blast-off, and (11) disposing of or recycling the used mold. Slurrying is usually done in a Hogue machine (a screw-type blender).
The fibrous-talc containing molding composition meets the various requirements needed of the material at each step. It is easily dry blended. The materials are relatively inexpensive. Slurry characteristics are good, making mixing in a Hogue machine possible. Setting times are fast. The wet and dry mold features are acceptable in terms of density, strength, permeability, shrinkage, and penetration resistance. In addition, the mold surfaces produced with such compositions are of generally good quality.
Despite all of this, gypsum-fibrous talc-sand molding compositions can no longer be used in making metal castings molds. The reason is that the fibrous talc has been removed from commerce because it is allegedly objectionable as a health hazard under recently enacted OSHA regulations. The need thus exists for a suitable fibrous talc-free replacement for such molding compositions.
One possibility is use of a composition marketed by Georgia Pacific Corp. of Portland, Oregon. It utilizes approximately 74% gypsum plaster, 12% wollstonite, 9% diatomaceous earth, 3% sand, and 2% calcium aluminate cement. The problem with this type of composition is that during slurrying in the Hogue machine, it does not achieve the consistency required quickly enough. Thus, the slurry poured onto the master is too thin so that material segregation occurs due to settling with resultant adverse affects on the quality of the plaster mold itself and metals cast into it. Of course as the plaster hydrates and sets up, the slurry thickness is increased to the point where a proper consistency is achieved, but by this time it has lost its usefulness relative to its use in the continuous commercial mixing machines.
Accordingly, there remains an urgent need to develop a fibrous talc-free molding composition having characteristics at least as good as those of the fibrous talc-containing molding compositions. The urgency comes from the fact that the previous sources of supply of fibrous talc are no longer usable.